I was trying to build a model by importing from a jdbc source. The first time thru the import I selected a couple of the tables (there are a couple hundred) and created the model. The next time thru the import, I wanted to add to the model, those tables not already in the model. I selected the "Update (If existing model is selected)" option, expecting it to update the existing model with the newly selected tables. However, it removed my existing tables, with the model only containing the newly selected tables. The only way around this is to always select what was previously in the model. I think the "Update" option is a misnomer, maybe is should be called "Replace".